All Monday flight operations at the Philadelphia International Airport have been canceled, an airport spokeswoman said.

Delta said flights from Washington to Boston will stop Monday morning. The airline had already canceled all flights out of New York and Philadelphia starting Sunday evening.

United Airlines canceled about 3,700 flights between Sunday and Wednesday.

Both companies are allowing some customers to change their flight plans without paying any fees due to Sandy.

Delta will let those ticketed to fly between Sunday and Wednesday — to and from airports in 15 states and the District of Columbia — to reschedule by November 4. United’s offer applies to travel to and from 29 airports, for the same dates.

Other airlines, such as American, are offering a similar process, with slight variations.